This invention relates to an apparatus and method for making individual flexible web products, and, in particular, to an apparatus and method for producing bags of different widths from a folded web of heat sealable thermoplastic material.
In the production of individual flexible web products such as plastic bags and containers, the bag stock is typically supplied in the form of a continuous web of thermoplastic material which has been folded upon, itself to form two plies. In forming individual plastic bags and containers, portions of the thermoplastic material are severed from the web. These severed areas also become side seams for the bags because they are typically sealed at the same time as they are severed by the use of a heated wire element. The plastic bags are then stacked, counted, and packaged by packing equipment.
The severing and sealing operation typically takes places on a relatively large diameter rotating drum which may contain multiple heated wire severing and sealing elements positioned in grooves located within the outer periphery of the drum. See, for example, Tumminia Patent No. 4,369,449, assigned to the same assignee as the present invention. As the drum rotates through a given arc, different severing and sealing elements are actuated and raised up to the drum surface to sever and seal a respective portion of the web of bag stock. The individual bags are retained on the drum by a vacuum arrangement as the drum rotates. Such drums are large and expensive pieces of equipment.
Individual bags are then taken from the drum, stacked, and packaged. See, for example, U.S. Pat. Nos. Re 28,172, 3,254,889, 3,599,705, 3,640,050, and 3,842,568, for a description of typical stacking and packing apparatus. Desirably, the packaging operation occurs at the highest possible speed at which the equipment can be operated to increase productivity of the system. As shown in the above mentioned patents, presently, individual bags are taken from the drum by a smaller drum, also suitably equipped with vacuum capabilities. The vacuum on the bags on the large drum is relieved at an appropriate point, and the bags fall onto the smaller drum where they are held in position by vacuum. At an appropriate point, the vacuum is released and the individual bags are pulled off the smaller drum by an orbital packer or similar device.
Early rotary drum apparatuses provided for cutting bags at fixed intervals on a smooth drum, e.g. Allison, U.S. Pat. No. 2,737,859, permitting production of one bag size. Since then the versatility of bag making machines has been improved to permit the production of a variety of bag widths on one machine. Improvements in mass production rates have also been sought. The machines disclosed in U.S. Pat. Nos. 3,901,754, 4,115,183 and 4,331,502 use two sets of support rollers which separate and tuck the web of thermoelastic material radially inward into a zig-zag shape before cutting and sealing along one set of spring-mounted supports. Bag width can be adjusted by changing the separation between the two sets of support rollers. In the '754 machine, the supports are fixed on two nested drums. Bag width is changed by adjusting the axis of rotation of smaller drum 15, thereby altering, within limits, the relationship between the rods and bars which separate from a point of intersection between the drums to form the tuck. Bag width may also be varied by removing intermediate rods or bars. In the '183 and '502 machines, where both sets of bars are on the same drum, bag size may be varied only by changing the cam plate which controls the motion of bars which are drawn radially inward by spring tension to form the tuck. While these machines permit continuous bag production at high rates, their drawback is that they have many moving parts.
U.S. Pat. Nos. 4,557,713 and 4,609,367, employ adjustable tuck bars to tuck the web material radially outward rather than inward. The depth of tuck and tension relief is controlled by a cam track. No springs are used to effect tucking or tension relief in the web. Infinite variation of bag size is possible within dimensional limits of these machines by adjusting the cam track which controls tuck bar movement. As the product drum rotates through a complete cycle, the tuck bars extend radially outward to tuck the web, and retract inside the drum periphery to allow product transfer. Again, however, the '713 and '367 machines also have the drawback of many moving parts which can wear and require maintenance. Precision in bag width, therefore, continues to be of concern. Additionally, transfer and stacking problems occur with larger bag sizes on the '713 and '367 machines due to instantaneous acceleration of the bags at the point of transfer between the product drum and transfer drum, a problem discussed in more detail below.
When the thermoplastic material is severed and sealed, the resulting bag seams remain hot even as the bags pass from the large product drum to the transfer drum. Thus, when looped or tucked material is removed from the product drum and laid flat on the transfer drum, the bag seals may overlap and reweld. One solution has been, for example with U.S. Pat. Nos. 4,557,715 and 4,608,367, to increase the rotational speed of the transfer drum to create space between the bag seams on the transfer drum. However, because the transfer drum is required to rotate at a tangential velocity higher than that of the seal drum, the bags are subjected to instantaneous acceleration at the point of transfer. Transfer and stacking problems result, particularly with larger sized bags.
Also presenting bag sealing problems is the tendency of the thermoplastic material to melt back when cut by the hot sealing wires. It has been found that relieving tension in the thermoplastic web material permits the bag welds to seal more completely. New means to provide such relief have been sought. Thus, for example, cam tracks have been designed to cause the tuck bars of U.S. Pat. Nos. 4,557,713 and 4,609,367 and supports of U.S. Pat. No. 4,331,502 to retract slightly at the time of sealing to provide tension relief and improve the bag weld seal.
The need continues, therefore, for high volume production machines which achieve size adjustment with a minimum of moving parts. The wear on numerous moving parts presents problems in maintaining bag width precision, general maintenance, and repair. Sealing problems have been addressed by providing tension relief through machine design, and by maintaining weld separation between bags. The need to maintain bag weld separation requires further development to avoid transfer and stacking problems.